The present invention relates to an eyepiece lens system applicable to a viewing optical systems having relatively a wide viewing angle, such as astronomical telescopes and binoculars.
The eyepiece lens system is generally arranged between a real image formed by an objective lens system and an eye of an observer, or between a displaying element such as an LCD or CRT and the eye to magnify the real image or the displayed image. The eyepiece lens system should be constructed such that an exit pupil thereof is located outside the lens on the eye side in order to provide a sufficient eye relief. Therefore, in the eyepiece lens system, coma, curvature of field, and lateral chromatic aberration tend to occur relatively easily.
A Ramsden eyepiece lens system has been conventionally known as an eyepiece lens system that has a simple structure and is inexpensive. The Ramsden eyepiece lens system is constructed such that two plano-convex lenses having the same focal length are arranged to be apart by a distance equal to the focal length thereof, with making the convex surfaces thereof face each other. In the Ramsden eyepiece lens system, accordingly, the lateral chromatic aberration can be suppressed theoretically.
However, the Ramsden eyepiece lens system provides a relatively short eye relief, and the eye of the observer focuses on a planar surface of the piano-convex lens arranged on the object side. Therefore, dust or a flaw on the planner surface of the lens may be conspicuous in the field of view. In order to reduce such drawback, conventionally, when the Ramsden eyepiece lens system is used, the focal lengths of the two lenses are made different and/or a distance between the two lenses may be changed so as not to satisfy the above-described condition.
FIG. 1 shows a lens diagram illustrating a modified Ramsden eyepiece lens system which has been conventionally used. An object is located on a left-hand side of the drawing. The eyepiece lens system shown in FIG. 1 has a first plano-convex lens 1, and a second piano-convex lens 2, both having the same focal length and convex surfaces thereof facing each other.
Table 1 indicates the numerical structure of the eyepiece lens system shown in FIG. 1. In Table 1, ER denotes a diameter of an eye ring, B denotes an angle of emitted light with respect to an optical axis of the eyepiece lens, f denotes a focal length of the eyepiece lens system, r denotes a radius of curvature of each surface of lenses of the eyepiece lens system, d denotes a distance between surfaces (i.e., a distance between lenses or a thickness of the lenses), n denotes a refractive index with respect to a d-line (588 nm), v denotes an Abbe's number of each lens. S indicates that a field stop is provided. It should be noted that in this example of Table 1, a distance d2 between two lenses 1 and 2 is shorter than the focal length f of the entire eyepiece lens system.
TABLE 1 ______________________________________ ER = 2.0 mm B = .+-.20.0.degree. f = 10.01 mm ______________________________________ Surface Number r d n .nu. ______________________________________ S -- 4.82 1 -- 2.00 1.49176 57.4 2 -8.000 6.10 3 8.000 2.00 1.49176 57.4 4 -- ______________________________________
FIGS. 2A-2D show aberration diagrams of the eyepiece lens system shown in FIG. 1. Specifically, FIG. 2A shows spherical aberration for d-line, g-line and C-line, FIG. 2B shows a lateral chromatic aberration for d-line, g-line and C-line, FIG. 2C shows astigmatism (S: sagital; and M: meridional), and FIG. 2D shows a distortion. A unit of measurement of a horizontal axis for FIGS. 2A-2C is "mm", and that for FIG. 2D is "%".
If the afore-mentioned condition of the Ramsden eyepiece lens is modified to elongate the eye relief as is done in the above example, the lateral chromatic aberration increases as shown in FIG. 2B. That is, in the above-described conventional eyepiece lens system shown in FIG. 1 which is intended to elongate the eye relief sufficiently, it is important to suppress the lateral chromatic aberration.